The German Offenlegungsschrift 21 23 766 relates to copolymers with a hydrophilic surface. They are composed of 75 to 92% methyl methacrylate and 8 to 25% of acrylic acid. The polymerization takes place by a free radical mechanism in the absence of any solvent, so that polymers with a statistical distribution of the monomer units are obtained. The copolymers admittedly are referred to as block copolymers. However, this is a solventless copolymerization of the mixture of monomers and not the blockwise arrangement of the monomer units. The copolymers are recommended particularly for the production of contact lenses, since the surfaces of the shaped copolymers are superficially hydrophilized by reaction with bases.
Polymethyl methacrylate/poly-t-butyl(meth)acrylate block copolymers and their synthesis by anionic polymerization are described in Macromolecules, (1991), 24, 4997-5000. However, this method can hardly be carried out on an industrial scale, since the anionic polymerization requires very pure, anhydrous solvents (particularly tetrahydrofuran) and reactants. The reaction must be carried out at low temperatures of about -78.degree. C. Moreover, it is necessary to use alkali metal initiators.
The EP publication 0 518 225 discloses aqueous, pigmented inks for ink jet printers. These inks contain an AB or ABA block copolymer as stabilizer, the A segment of which is synthesized, for example, by polymerizing n-butyl methacrylate and the B segment initially by polymerizing trimethylsilyl methacrylate. From the latter, a polymethacrylic acid segment can be produced by splitting off the trimethyl group hydrolytically. For the polymerization, the group transfer polymerization (GTP method) described, for example, in U.S. Pat. No. 4,508,880, is employed.
A detailed description of this GTP may be found in Eur. Polym. J. Vol. 29, No. 2/3, 407-414 (1993). Typically, the polymerization is initiated by a O-silyl ketene acetate; 1-methoxy-1-(trimethylsiloxy)-2-methylpropene is customary. A nucleophilic compound, such as tris(dimethylamino)sulfonium bifluoride is used as a catalyst. The GTP method proceeds as follows: ##STR1##
As with the anionic polymerization, this method also requires that the reactants used should be dried very carefully and be free from residues of alcohol. Furthermore, even traces of moisture must be kept from the reaction. Moreover, the compounds used as initiator and as catalyst are expensive, so that this method, as a whole, is costly when used industrially.
The present invention is concerned with the technical problem of synthesizing block copolymers, the segments of which are formed from methacrylate esters and methacrylic acid or its salts. The desired block copolymers are to be synthesized in the simplest way possible, in order to enable these compounds to be made on an industrial scale.
Admittedly, the reaction of polymethacrylate esters with a large excess of diethylaminoethanol at temperatures in excess of 150.degree. C. in the presence of titanates as catalysts is known from Chemical Abstracts 98 (No. 8, 54652f). Polymers are obtained here, for which a portion of the ester groups is transesterified with diethylaminoethanol. However, relatively impure products are obtained at this temperature since it is impossible to avoid obtaining the thermal decomposition products.
Surprisingly, however, it was found that by adhering to certain structural parameters for the polymethacrylate esters that are to be reacted, and by selecting suitable process conditions for the transesterification, it is possible to transesterify one or both terminal ester groups, the terminal tertiary ester group being decisively preferred over the quaternary ester group. This enables the synthesis of novel polymethacrylate esters, the ester groups of which in the .alpha. and optionally in the .omega. positions, differ from those ester groups in the chain.